This research focuses on the nature and etiology of problem aggression in young children. In one study the characteristics of children's aggression in the home are assessed between 1 and 2 1/2 years of age. In a second study specific hypotheses concerning situational, familial, and dispositional determinants of aggression are tested on the basis of a series of laboratory procedures administered at two and again at five years of age. Also planned is a third study focusing on the aggression of very young children who have been selected because of problem aggression. The early existence of stable patterns of aggression among young boys was found. This pattern appears to be characterized by consistently above average aggression across multiple settings, intense patterns of aggressive behavior, emotional lability, and a greater responsiveness to the introduction of psychological stresses into the environment. Results suggest early childhood may be an important period for preventative and remedial intervention.